A known technique of closing ampoules, vials, and similar glass vessels which have been filled with materials is by sealing the open end in a flame. In this manner, the contents of the vessel are protected, until their use, against impurities, evaporation or sublimation.
Reopening of such hermetically sealed glass vessels in the easiest manner possible is fraught with many problems. The most usual method of reopening such sealed glass vessels consists of breaking the neck of the vessel, after a fracture line has been scratched with a file at the desired point in the glass surface. This method has the disadvantage that, when the ampoule neck is broken off, fine glass splinters are produced which contaminate the ampoule contents. Moreover, the person who opens the ampoule can be injured.
German Pat. No. DE-PS 867,291 teaches applying a substance, e.g. fused enamel, at a predetermined location along the line of fracture for the breaking of an ampoule. The coefficient of thermal expansion of the applied substance is larger than that of the ampoule glass, so that a tensile stress arises in the glass upon cooling of the applied substance, which favors a break at this location. This enamel fusion process is, as can be easily ascertained, very labor-intensive and leads to surface cracks due to the difference thermal expansion of the materials.
In accordance with another process disclosed in British Pat. No. GB-PS 762,857, a transverse notch is made on the neck of an ampoule by means of scratching, cutting, etching, or some other similar manner, and the glass is thus weakened at this location (predetermined breaking point). Since the degree of damage to the glass surface cannot be determined precisely, this process has the disadvantage that the ampoule can break prematurely.
It has also been proposed to solidify stresses at a predetermined breaking point by means of heating and cooling the vessel, and to open the vessel by damaging the glass at this breaking point with a suitable tool. A process is described in German Patents DE-PS 554,146, DE-PS 595,725, and DE-PS 683,259, in which a cold tool is brought into contact with the still plastic glass at the prescribed breaking point during the moulding of the vessel. In this way, stresses are produced in a predetermined zone of the glass side. Contact of the ampoule interior wall with a cooling tool for the purpose of producing stress is, even with current manufacturing methods, not realizable at all. The vessel can be opened by means of superficial scratching at a spot of this stress zone. More precisely, in the process specified in these three German Patents, a stress ring is formed that runs around the perimeter of a glass vessel, for example an ampoule, and this stress ring is composed of alternating tensile stress points and compressive stress points. This process has the disadvantage that, when the still plastic glass is brought into contact with the cold tool, surface defects arise, such as microcracks, weld points, breaks or other similar defects, with the result that a surface-damaged glass vessel is produced which can break prematurely. Moreover, the process specified in these three German Patents is problematic.
In order that a crack running around the entire perimeter of the ampoule can form at a subsequent superficial scratching, the alternating compressive and tensile stresses must be high enough, and the interval between two stress points must not be so large that the crack comes to a stop. These conditions are met with such difficulty that this process, although it looks promising, has never found acceptance since it was proposed over fifty years ago.